Anyone looking for a reason to be more active can start by worrying about their brain.

That's the conclusion of researchers behind a study out today showing people who burn off the most energy have healthier, younger brains compared with adults who do less.

Lead author Cyrus Raji of UCLA says the researchers set out to determine how physical activity is associated with gray matter during aging. Gray matter, also called the cerebral cortex, gets the credit for processing much of the information we use. It is made up of neurons and nerve fibers, and its shrinkage is a possible cause of or contributor to dementia and Alzheimer's disease, a fatal degenerative illness that affects 5.2 million adults in the USA. The number of diagnoses is expected to triple by 2050.

In the study of 876 adults (ages ranged from 69-95), those who burned the most calories had 5% more gray matter. The findings were presented at the annual meeting of the Radiological Society of North America in Chicago. The researchers used MRIs and 3-D pictures of the brain to measure volume.

"If you want to maximize the effect on your brain, these physical activities are something people have to start engaging in earlier in life, in your 50s and 40s," says Raji.

Those in the top 25% for physical activities burned off 3,434 calories a week compared with those in the bottom 25%, who burned only 348 calories a week. It takes about 110 minutes to walk off 560 calories.

Though the study shows a link between activity and brain health, the findings don't prove that activity preserves brain matter, says Dave Knopman, a spokesman for the American Academy of Neurology.

"It could be the people with the bigger brains are more physically active," says Knopman, a neurology professor at the Mayo Clinic in Rochester, Minn.

The patients' conditions ranged from normal cognition to Alzheimer's dementia. Benefits were also noted in people who have mild cognitive impairment and Alzheimer's disease, Raji says. Greater caloric expenditure was related to larger gray matter volumes, including the hippocampus.

While noting that biological changes in other parts of the brain (beyond gray matter) are also possible causes for dementia, Joshua Willey, a neurologist at Columbia University, says this study's focus on regions of the brain is encouraging.

"We care about the volume of the hippocampus because that seems to be the first area that's affected in Alzheimer's disease," Willey says. "That's the part of the brain most of us think of in terms of short-term memory. So this is exciting work."

A 2011 study of 120 previously sedentary adults ages 55 to 80 found that those who walked around a track for 40 minutes three days a week for a year increased the volume of their hippocampus. Older adults assigned to a stretching routine showed no hippocampal growth.

Walking is aerobic in nature, making the heart, lungs and large muscle groups work harder than at rest.

"Virtually all of the activities examined in this study are some variation of aerobic physical activity, which we know from other work can improve cerebral blood flow and strengthen neuronal connections," he says.

Knopman gives the aerobic theory a thumbs-up: "People who are active are also less likely to have diabetes, high blood pressure, are less likely to be obese or have heart disease, all conditions associated with an increased risk of getting dementia."